linux-kernel - [tip: locking/core] locking/qspinlock: Use atomic_try_cmpxchg_relaxed() in xchg

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Message-ID: <171284242106.10875.12200696780479010207.tip-bot2@tip-bot2>
Date: Thu, 11 Apr 2024 13:33:41 -0000
From: "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
 Waiman Long <longman@...hat.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/qspinlock: Use
 atomic_try_cmpxchg_relaxed() in xchg_tail()

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     79a34e3d8411050c3c7550c5163d6f9dc41e8f66
Gitweb:        https://git.kernel.org/tip/79a34e3d8411050c3c7550c5163d6f9dc41e8f66
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Thu, 21 Mar 2024 20:52:47 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Thu, 11 Apr 2024 15:14:54 +02:00

locking/qspinlock: Use atomic_try_cmpxchg_relaxed() in xchg_tail()

Use atomic_try_cmpxchg_relaxed(*ptr, &old, new) instead of
atomic_cmpxchg_relaxed (*ptr, old, new) == old in xchg_tail().

x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after CMPXCHG.

No functional change intended.

Since this code requires NR_CPUS >= 16k, I have tested it
by unconditionally setting _Q_PENDING_BITS to 1 in
<asm-generic/qspinlock_types.h>.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Waiman Long <longman@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20240321195309.484275-1-ubizjak@gmail.com
---
 kernel/locking/qspinlock.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index ebe6b8e..1df5fef 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -220,21 +220,18 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
  */
 static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
 {
-	u32 old, new, val = atomic_read(&lock->val);
+	u32 old, new;
 
-	for (;;) {
-		new = (val & _Q_LOCKED_PENDING_MASK) | tail;
+	old = atomic_read(&lock->val);
+	do {
+		new = (old & _Q_LOCKED_PENDING_MASK) | tail;
 		/*
 		 * We can use relaxed semantics since the caller ensures that
 		 * the MCS node is properly initialized before updating the
 		 * tail.
 		 */
-		old = atomic_cmpxchg_relaxed(&lock->val, val, new);
-		if (old == val)
-			break;
+	} while (!atomic_try_cmpxchg_relaxed(&lock->val, &old, new));
 
-		val = old;
-	}
 	return old;
 }
 #endif /* _Q_PENDING_BITS == 8 */